Black smoke billows up into the sky following an explosion at the Chemiepark in Marl, western Germany March 31, 2012. [Photo/Agencies]

BERLIN - A huge explosion rocked a chemical industrial park in Marl, western Germany, leaving one or two workers injured and three missing, police said Saturday.

Marl police confirmed a pressure vessel in the park blasted at 13:36 local time (1136 GMT).

Three staff members were missing, police spokesman Michael Franz told Xinhua. He said one was injured, although another spokesman said earlier two had been confirmed injured.

The firefighters were "fully engaged" and the flame was "locally limited," the police said.

The accident in the park in the Ruhr industrial region sent up a giant gas cloud, which moved southward and was spreading over a major east-west highway, the police said.

Franz said emergency crew had tested the cloud at the site and found no harmful content. Police have warned local residents to close windows and doors and stay inside buildings.

There is no death report yet and the cause of the explosion is under investigation.

The chemical industry facility is operated by the company of Infracor. It is one of the biggest German chemical parks, covering an area of about 6.5 square km and containing more than 100 chemical plants from about 30 firms.